In gas turbine engines, compressor blades are typically supported on a compressor disc by a dovetail root fixing, as shown in FIG. 1.
The dovetail root fixing has a neck portion 100 at the base of the aerofoil body portion 102 of the blade, and a radially inwards dovetail portion 104 which extends outwardly from the neck portion to provide angled (typically 45°) support flanks 106 at either side of the fixing. The fixing is joined to the compressor disc 108 by sliding along a correspondingly-shaped slot 110. To allow assembly of the blade in the disc, a clearance C is provided between the neck portion and the disc slot. When the disc spins, the blade is centrifuged radially outwardly and the support flanks 106 engage with matching angled surfaces 112 of the slot to support the blade in the disc.
There are typically two approaches for assembling compressor blades onto a compressor disc. The first is to provide a row of slots around the circumference of the disc (one of which is shown in FIG. 1), with each slot being loaded with a respective blade. The slots may be axial (i.e. parallel to the engine centre line) or skewed axial (i.e. angled to the centre line but nonetheless with a significant axial component). The second approach is to provide a circumferentially extending slot around the circumference of the disc, and to load a row of blades into the slot. FIG. 2 shows schematically a longitudinal cross-section through a compressor section of a gas turbine engine, and illustrates an axially-loaded compressor blade 114 and a circumferentially-loaded compressor blade 116.
To enable tight tolerance control of the tips of all the compressor blades, the tips can be ground using high speed grinding. This involves rotating the assembled rotor, and grinding the blade tips with a rotating grinding wheel. The tip tolerance is measured during the grind, and tolerances to a few microns can be achieved. The complete rotor can then be mounted into the engine.
However, when the rotor spins and the blade centrifuges outward, each blade adopts a position within a range of tilt angles permitted by the neck clearance, as illustrated by FIG. 3 which superimposes the blade positions on the cross-section of FIG. 1 for the two extreme tilt angles of the blade, one tilt angle eliminating the left hand clearance C and the other eliminating the right hand clearance C. Similarly, FIG. 4 shows schematically three superimposed positions of a compressor blade relative to its circumferentially extending compressor disc slot, namely: a forwarded tilted position, a non-tilted position and a rearward tilted position. The same forward-to-rearward tilting effect arises in blades mounted in skewed-axial slots as the slots are angled to the centre line. For the small angles of blade tilt possible within a typical root neck clearance, the friction within the dovetail root fixing is sufficient to lock the blade in the position it ends up in when the spool is rotated up. This position is effectively random and changes for every blade on every spool start up.
The forward-to-rearward tilting can cause errors in the blade tip grinding process if the rotor is stopped and restarted (e.g. when a damaged blade is replaced). More significantly, during engine running, the tilting can cause deeper rubs in the casing liner above the blade leading edge (LE) & trailing edge (TE) tips, than above the middle of the blade, creating an M-shaped rub. This has implications for compressor efficiency. In addition, the LE and TE rubs, if deep, can lead to cracking in the tip of the blade, and ultimately the loss of a blade tip corner.
FIG. 5 shows the effect of blade tilt on blade tip height. Because of blade tilting, the blades in a set may exhibit variations in height even after grinding. Moreover, if a blade is tilted fully anti-clockwise 101a in the root when it is ground, the blade will be X1 shorter than a blade that was centralised 101b during grind. If this blade then rotates fully clockwise 101c during a subsequent run in the engine, the blade tip will be (X1+X2) radially further outward that the nominal desired tip position. This will result in the blade tip rubbing out the abradable liner by an additional (X1+X2) depth, which increases the blade tip running clearances by (X1+X2) for a blade with a nominal blade tip position. For a blade that tilted clockwise during grinding but then tilts anticlockwise during running, the blade tip will be (X1+X2) radially more inward than a nominal tip. If this blade is running within a casing that has suffered the (X1+X2) additional rub, the additional tip clearance this blade will see is 2(X1+X2). The overall effect on running clearances and hence compressor efficiency and surge margin may be significant.
Another unfortunate effect of blade tilt associated with axial or skewed-axial disc slots, is the effect it has on the angle of the tip of the longest blade. In the example shown in FIG. 5 the rotor rotational direction is clockwise i.e. from left to right in the diagram. The blade with the highest tip, as explained above, will have rotated clockwise by φ° relative to the position it was in when it was ground. As a result, the blade tip will have a negative relief (or clearance) angle with the casing of φ°. This means its suction surface edge will touch the casing rather than the pressure surface edge. However, the blade tip will be a relatively inefficient cutting tool, due to the negative relief angle at this edge. As a consequence, for high incursions significant heat and blade vibration may be generated, causing over-cutting of the soft abradable liner due to heat build-up and aerofoil cracking due to vibration. Further, if the liner material heats up to above its maximum operating temperature, the material can soften and accumulate on the longest blade (which is doing the rubbing). This accumulation in turn makes the blade longer, causing it to machine out the liner around the complete circumference. All the other blades then run with a much increased tip clearance.